CONTKIBUTIONS FROM THE WISLEY LABORATORY. 



47 



been deduced therefrom that an improvement in the condition of the 

 strain of the variety used was effected. It would be unwise, however, 

 from the results of so few experiments, to suggest the practice of sending 

 " seed " diseased in this way to another locality with the object of 

 eliminating the disease. All experiments of this kind need to be repeated 

 year by year to be increasingly valuable. Nevertheless, there are 

 secondary considerations which are in harmony with the narrower 

 results. It is significant that the affected tubers are difficult to obtain 

 in the autumn; it is not until the middle of, or late in, the storage 

 season that the disease becomes unpleasantly evident. The expression 

 of any constitutional weakness, from whatever cause derived, might be 

 expected to develop in this period. Again, it is noteworthy that, 

 although the soils used in the experimental plots were relatively poor, 

 yet this particular disease did not occur on the land in question in the 

 two years following that in which the original experiments were 

 conducted. If this result be compared with others obtained by the 

 writer from planting diseased tubers affected with internal disease, 

 streak-disease and Phytophthora, this difference is noteworthy — that 

 whereas in the former case all the sets produced healthy plants, in the 

 latter case many sets failed to develop, and disease appeared in the 

 crop. 



The above-mentioned observations lead to a discussion of the probable 

 cause of bruise. It has been pointed out that the earliest stage of the 

 disease involves the death of the storage-cell, as indicated by a change 

 of the refractive index of the protoplasm and by the fact that the 

 protoplasm ceases to move. Subsequently the dead protoplasm changes 

 slowly, until the original structure is obscured. The earliest changes 

 are not associated with the presence of any hyphal organism within 

 either the cells or the intercellular spaces, so that the direct injury to 

 the tissue must be due either to bacteria or to some physiological 

 circumstance. Several obscure diseases have received a purely physio- 

 logical explanation — after repeated failures to isolate definite pathogenic 

 bacteria. — such as, for instance, the Mosaic disease of tobacco, which 

 is said to be due to a disturbance of the normal physiological activity 

 of the cells of the affected plant, owing to cutting back." A few 

 years ago BidgoodI suggested a theory of the cause of certain spot- 

 diseases of Calanthe leaves based upon observations made by Schunk| 

 upon the presence of an unstable glucoside in the leaves of Phajus and 

 Galanthe. Schunk found that the glucoside in question (indican) 

 decomposes spontaneously with the production of insoluble indigo in 

 the cells as soon as the vitality of the cell is destroyed. Bidgood's 

 suggestion is that the decomposition of indican with the production of 

 insoluble indigo, owing to a lowering of the cell's vitality, may be a 

 cause as well as an effect of the disease, since the glucoside is so 

 unstable that the liberation of indigo may easily be induced artificially. 

 If Mandelin's reaction and Brandt's reaction be considered reliable 



* Woods. U.S. Dept. of Agric. Bur. of Plant Ind. Bull. 18 (1902). 

 t John Bidgood. Jour. Roy. Hort. Soc. (1904-5), p. 126. 

 I Schunk. Annals of Botany (1897), p. 439. 



